Photosynthesis: using light energy to reduce
CO2 to make CH2O (carbohydrate)
Photosynthesis light reactions
• How do photosynthetic organisms capture light
energy?
• What reactions convert light energy to chemical
energy?
Light reaction take-home points
• Light energy is used to generate a proton gradient
across a membrane for chemiosmotic ATP synthesis.
• Light energy is used to reduce electron carriers that are
used, in turn, to reduce inorganic carbon (CO2) to
make carbohydrate (CH2O).
• A key innovation enabled cyanobacteria to take
electrons from water molecules instead of organic
molecules or sulfur and changed the surface of the
planet forever.
Oxygenic Photosynthesis
• 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2
• Reverse of aerobic respiration:
C6H12O6 + 6 H2O + 6 O2  6 CO2 + 12 H2O
Evolution of chloroplasts from
cyanobacterial endosymbiont?
http://evolution.berkeley.edu/evosite/history/endosym.shtml
Visible light is a portion of the EM spectrum
Light is energy: E = h
Bacteria and chloroplasts use chlorophyll to
capture light energy
Xiong et al. 2000
Science 289:1724-1730
Chlorophylls and other pigments form a lightharvesting complex in photosynthetic membranes
Absorption spectra for chlorophylls match
photosynthetic action spectrum
Campbell & Reece, 7th ed.
Light-harvesting complexes funnel light
energy to a photosystem reaction center
Light-activated reaction center chlorophyll molecules give up
electrons (are oxidized)
Noncyclic photophosphorylation (Z-scheme)
uses two photosystems: PSI and PSII
Light + H2O + NADP+ + ADP + Pi  NADPH + H+ + ATP + 1/2O2
Energy diagram of non-cyclic electron flow
Protons are pumped into the thylakoid
interior
Campbell & Reece, Biology 8th ed.
Chemiosmotic ATP synthesis in chloroplasts
resembles that in mitochondria
Electron transport chains
are located on the
thylakoid membrane and
inner mitochondrial
membranes.
Campbell & Reece, Biology 8th ed.
Summary of Z-scheme (non-cyclic)
• PS II
– P680
– Gets electrons
from:
• PS I
– P700
– Gets electrons
from:
– Gives electrons to:
– Gives electrons to:
– Products:
– Products:
Cyclic photophosphorylation by PSI makes
ATP but not NADPH
Campbell & Reece, Biology 8th ed.
Fd(red) can reduce NADP+, or PQ (electron transport chain)